In Western Canada, a series of deformational events, probably the result of the accretion of a series of allochthonous terranes and accompanying subduction, have deformed the miogeoclinal sediment prism that previously formed the western margin of North America. The miogeoclinal prism was composed of a series of Proterozoic, Paleozoic, and early Mesozoic sedimentary successions, apparently deposited in response to an extended series of episodes of rifting and subsidence. Tectonism leading to the development of the Western Canada foreland basin commenced in the Middle Jurassic, apparently the result of the initial collision between North America and the allochthonous terranes to the west. Various clastic wedge successions were shed eastward into the foreland basin through ate Jurassic to Paleocene, but at present they can only be loosely linked to identified tectonic episodes in the interior of the Canadian Cordillera. Transpressional tectonics, and deposition of foreland basin sediments, terminated at about the Paleocene-Eocene boundary as a change in tectonic regime produced large-scale extension in the interior of the Cordillera that lasted through the mid-Tertiary.

The foreland fold and thrust belt, developed along the eastern margin of the cordillera, overlaps and deforms sediments of the foreland basin. For much of its length through Canada, the foreland belt exhibits a thin-skinned structural style. The North American craton, essentially undeformed, extends westward beneath detached, horizontally compressed, and tectonically thickened strata of the miogeocline, the cratonic platform cover, and the foreland basin. Thrust faulting, with associated folding, was the dominant deformation mechanism in the foreland belt. The style of thrusting, and its relationship to folding, were influenced by several factors such as the geometry and

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composition of the deformed sedimentary sequence, the magnitude of shortening, the lateral extent of the basal detachment, and possibly pore-fluid overpressures.

The Foothills belt is a significant source of hydrocarbons. The major reservoir units are Devonian, Mississippian, and Triassic dolostones, all rocks belonging originally to the miogeoclinal succession. Hydrocarbon gases and oils filling the reservoirs were probably produced around the time of greatest burial, which broadly coincided with the time of maximum thrusting (Late Cretaceous-Paleocene).